Poor control of blood sugar increases the risk of kidney disease or damage, known as diabetic nephropathy.

Bariatric surgery had better than expected results on these kidney problems, says researcher Philip R. Schauer, MD, director of advanced laparoscopic and bariatric surgery at Cleveland Clinic, where the study was conducted.

"We expected it to improve or just slow down [the kidney disease]," he tells WebMD. "Surprisingly, a significant percent of patients who had diabetic nephropathy actually had remission."

In the study of 52 patients, over one-third had diabetic nephropathy before the surgery, says researcher Helen M. Heneghan, MD, a bariatric surgery fellow at Cleveland Clinic who presented the findings.

Five years after the surgery, the kidney disease resolved in 58% of these patients, Heneghan says.

In addition, five years after the surgery, 44% of the patients had remission of their diabetes and one-third had improvement, she says.

''This study needs to be validated," Schauer tells WebMD. However, he says the results make sense.

There is now convincing evidence that weight loss surgery can improve blood sugar control in those with diabetes, he says.

"It makes sense if we can control the blood sugar more effectively, we can control kidney problems as well," he says.

Weight Loss Surgery & Kidney Disease: Study Details

The 52 patients had been diagnosed with type 2 diabetes for an average of nearly nine years. Their body mass index or BMI before surgery averaged 49 (a BMI of 30 or more is considered obese).

Most patients had gastric bypass surgery. A surgeon creates a stomach pouch out of a small portion of the stomach and attaches it to the small intestine. A large part of the stomach and some of the small intestine are bypassed.

While 37% had diabetic nephropathy at the study start, the others were also at risk, statistics suggest.

Of the patients who didn't have nephropathy at the start, one-quarter did develop albuminuria, in which too much protein is found in the urine. This reflects kidney disease or other problems.

Compared to before the surgery, the patients also had improvements in blood sugar levels and in HbA1c, an indicator of blood sugar control over the previous three months.

Overall, Heneghan says, the surgery had good effects on the kidney problems as well as the diabetes. "We believe bariatric surgery can induce a significant and sustained improvement in type 2 diabetes and prevent complications such as diabetic nephropathy," Heneghan says.

About 200,000 U.S. adults have bariatric surgery annually, according to the American Society for Metabolic and Bariatric Surgery. The risk of death is about 0.1%. The major complication rate is about 4%.

Bariatric Surgery & Kidney Problems: Perspective

The study has good points and limitations, says Benjamin Canales, MD, MPH, assistant professor of urology at the University of Florida in Gainesville. He reviewed the study.

He conducts bariatric surgery research on animals. He believes the Cleveland Clinic study is the longest follow-up to date to look at how bariatric surgery might improve kidney problems.

"This is a very promising long-term study in a group of diabetic patients who typically progress in their kidney disease over a course of five to 20 years," he says.

One limitation, however, he says, is the lack of a comparison group. In this case, it would be obese patients with diabetes who did not have the surgery.

"The author's conclusion that bariatric surgery 'may improve or even prevent long-term nephropathy' may be true, there's just no way to make this assumption based on the small number of positive results presented here," Canales tells WebMD.

More study is needed, he says.

These findings were presented at a medical conference. They should be considered preliminary as they have not yet undergone the "peer review" process, in which outside experts scrutinize the data prior to publication in a medical journal.